The Yamaha YZ125 is a motorcycle with a 124.9 cc two-stroke engine designed for motocross riding and produced by Yamaha Motor Company. 1973: Although very few were actually made, Yamaha did produce a YZ125 in 1973. It was not offered to the general public, but a dealer in good standing could submit a rider resume to the importer and hope to obtain a semi-works bike. However rare, this goes down in the books as the first bike with the YZ125 name. For the first two years it was made with dual rear shocks (dirt bikes ever, due to their wide powerband and excellent handling. The YZ125 has been ridden to five AMA National Motocross Championships, and multiple AMA Regional Supercross Championships.
In 2001, Yamaha released a bike designed to complement the YZ125, the four-stroke YZ250F. The two bikes shared a rolling chassis and are eligible to compete in the same racing class. However, the YZ250F has a slight horsepower advantage.
The YZ125 has been built with five- or six-speed sequential gearboxes depending on model year. The 2005 model has a constant mesh wet, multiple-disc coil spring clutch.
The revised gear ratios as of the 2005 model are:
Gear ratio - 1st gear 31/13 (2.385)
Gear ratio - 2nd gear 29/15 (1.933)
Gear ratio - 3rd gear 27/17 (1.588)
Gear ratio - 4th gear 23/17 (1.353)
Gear ratio - 5th gear 24/20 (1.200)
Gear ratio - 6th gear 23/21 (1.095)
From 1973 through 2004, the YZ 125 had a single backbone frame made from steel. It generally averaged from 176 to 198 lb (80 to 90 kg). For the 2005 year, however, Yamaha switched to a single backbone frame constructed from an aluminum alloy. This frame material change dropped the weight to 190 lb. (dry weight). For 2008 models, the wheel assemblies and front fork suspension were redesigned, yielding additional weight savings, making wet weight, no gas sub-200 lb. Aluminum-framed YZ125s are notably "flickable" and sometimes this trait is seen as a drawback since they tend to become more difficult to control on rough surfaces.
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